Probing Question: What is the next big step in solar power?

Probing Question: What is the next big step in solar power?

Walt Mills

November 29, 2012

photos.com/Elena Elisseeva

Any idea how many watts of energy the world requires at any given moment? If you’re not guessing in the trillions, you’re way off. Experts say it takes an estimated 13 trillion watts of energy to power our economies, and by 2050 expect that number to increase to 30 terawatts of power.

Explains Jeffrey Brownson, assistant professor of energy and mineral engineering at Penn State, less than one-tenth of one percent of today’s electricity comes from solar photovoltaics, or PV, the panels on your neighbor’s roof that convert the energy of sunlight into electricity. But the path to 30 terawatts, he adds, almost certainly goes through the Sun, a source of nearly all energy on the planet and capable of providing 600 terawatts of continuous usable global energy.

Brownson, an expert in photovoltaics and sustainability, is one of a number of Penn State researchers pursuing solar-powered technologies, including photonic concentration, silicon nanowires, and dye-synthesized polymer and quantum dot solar cells. Despite great potential, these next-generation solar cells are still in early stages of development and are not likely to have an impact on the PV industry any time soon, Brownson cautions.

Most of today’s solar panels are made from some type of silicon, of either single- or multi-crystalline type, he notes. Single crystal silicon is cut from large boules of pure silicon and is the most efficient in terms of energy conversion among non-concentrating PV systems. It is also expensive. Multi-crystalline silicon is made from a melt of raw silicon that is cooled and cut into square wafers. Cheaper than single-crystal silicon, it is also less efficient and takes up more room on a rooftop.

Walt Mills

Solar energy researchers Rona Benai and Jeffrey Brownson

About five percent of commercial solar panels use thin-film solar cells, which are made by depositing thin layers of material on a substrate, adds Brownson. Current thin-film solar cells come in three “flavors,” he explains: amorphous silicon, cadmium telluride, and CIGS, which stands for copper indium gallium selenide. CIGS’ thin-film solar cells are the type that the controversial California solar cell company, Solyndra, produced before going bankrupt. “We have Solyndra solar panels in our lab,” says Brownson, who was the faculty advisor to Penn State’s 2009 entry in the Solar Decathlon. “They worked great. They were just too expensive.”

“For a while, cadmium telluride thin film solar cells took off, but now multi-crystalline silicon is the standard,” adds Brownson’s graduate student, Rona Banai, who is president of Penn State’s student chapter of the American Solar Energy Society. Prior to enrolling at Penn State Banai worked as a researcher in the photovoltaics industry. Still, she adds, cadmium telluride remains a viable and competitive alternative.

For places like Pennsylvania, thin films have a lot of advantages over silicon, says Banai. Silicon is less efficient at absorbing light on cloudy days and at low angles. Thin films reflect less light than silicon and are good on cloudy days. Thin films also use less material, are easy to manufacture, and have lower embodied energy, the amount of energy required to manufacture each solar panel. The downside is that both cadmium telluride and CIGS contain hard-to-come-by minerals, tellurium and indium, that are found primarily in China, which has its own booming photovoltaic industry. Also, the tellurium in cadmium telluride is rare, and the indium in CIGS, though more abundant, is in high demand for display screens. “They’re not cheap materials, and they’re getting more expensive,” Brownson says.

Brownson and Banai are looking instead at a cheap and widely available material they think could be the next big thing in solar cells, a thin film called tin sulfide. Both tin and sulfur are orders of magnitude less expensive than cadmium telluride. Their theoretical analysis shows tin sulfide should have up to 20 percent efficiency. And it is a better absorber than either cadmium telluride or silicon. They are currently testing the electrical properties tin sulfide thin films in collaboration with Mark Horn’s group in Electrical Engineering.

Though tin sulfide was studied and pretty much forgotten fifty years ago when solar research waned in the United States, Brown and Banai believe their material could have an immediate impact, transforming the field of photovoltaics with a cheap, easily manufactured thin-film made from a readily available source. Other university labs are starting to get onboard.

The question about affordability remains: When will solar photovoltaics become cost competitive? For decades the number of solar modules being manufactured has doubled every 2.2 years, Brownson says. Since 2010, the market has doubled every year. If you graph the falling cost of solar modules with the rising cost of electricity, some economists predict that the lines will cross in 2018, and renewable solar energy will compete with other energy sources. Brownson cites the number of large materials and glass companies moving into the market to bolster the claim.

“The funny thing about exponential growth, it tends to creep up on you,” he says. “Like cell phones—a few years ago, you were unusual if you had one. Now, people all over the world have them. Solar is starting to look like that.”

Jeffrey R. S. Brownson, Ph.D., is assistant professor of energy and mineral engineering and materials science and engineering. Contact him at nanomech@psu.edu. Rona Banai is a Ph.D. student in materials science and engineering. Contact her at reb285@psu.edu.